The lack of adequate X-ray observing capability is seriously impeding the progress in understanding the hot phase of circumgalactic medium (CGM), which is predicted to extend to the virial radius of a galaxy or beyond, and thus in acquiring key boundary conditions for studying galaxy evolution. To this end, the Hot Universe Baryon Surveyor (textit{HUBS}) is proposed. textit{HUBS} is designed to probe hot CGM by detecting its emission or absorption lines with a non-dispersive X-ray spectrometer of high resolution and high throughput. The spectrometer consists of a $60times60$ array of microcalorimeters, with each detector providing an energy resolution of $2~mathrm{eV}$, and is placed in the focal plane of an X-ray telescope of $1^{circ}$ field-of-view. With such a design, the spectrometer is also expected to enable studies of intra-group medium (IGrM) and the outer region of intra-cluster medium (ICM). To assess the scientific potential of textit{HUBS}, we created mock observations of galaxies, groups, and clusters at different redshifts with the tng simulation. Focusing exclusively on emission studies in this work, we took into account the effects of light cone, Galactic foreground emission, and background AGN contribution in the mock observations. From the observations, we made mock X-ray images and spectra, analyzed them to derive the properties of the emitting gas in each case, and compared the results with the input parameters from the simulation. The results show that textit{HUBS} is well suited for studying hot CGM at low redshifts. The redshift range is significantly extended for measuring IGrM and ICM.
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